const map = new Map();
map.set('first', 'hello');
map.set('second', 'world');

for (let [key, value] of map) { // 解构
  console.log(key + " is " + value);
}
// first is hello
// second is world
// 获取键名
for (let [key] of map) {
    // ...
  }
  
  // 获取键值
  for (let [,value] of map) {
    // ...
  }